In the discussion of the background that follows, reference is made to certain structures and/or methods. However, the following references should not be construed as an admission that these structures and/or methods constitute prior art. Applicant expressly reserves the right to demonstrate that such structures and/or methods do not qualify as prior art.
The recent development within tools has been towards sharper cutting edges. This requires thinner coatings in order to maintain the sharp cutting edge since a thick coating would result in an increased edge rounding which is detrimental for the cutting properties. A thicker coating is also more prone to spall off the edge line as a result of a higher interfacial shear force, which is more critical the sharper the edge is. If a thinner coating is deposited, an increased hardness, i.e., abrasive wear resistance, is required to maintain the wear resistance of the tool.
One way to achieve a thin, hard coating is to use a multilayered nanocomposite coating. A common coating composition for such a multilayered nanocomposite coating is (Ti,Al)N.
Another way to improve properties like hardness and oxidation resistance for PVD-coatings is by the addition of elements such as Cr, Si, B.
For Al-containing nitride PVD coatings, it is well known that a high aluminium content, >40 at %, is advantageous to obtain a high flank wear and oxidation resistance. However, a too high aluminium content also leads to increased crater wear due to the phase transformation from the metastable hard cubic phase to the more stable, soft, hexagonal phase of AlN.
Sharp edges are important in many cutting operations, e.g. in drilling operations. When drilling, wear is not only found in the cutting edge. Frequently, substantial wear is also found on the margin of the drill. Severe margin wear gives problems when the drill is going to be reconditioned, i.e., reground and recoated, as the whole worn area has to be removed. Normally when recoating a drill the total coating thickness increases for each recoating. An increased coating thickness on the margin will increase the wear on the margin, some distance away from the corner. Also, an increased coating thickness will increase the edge rounding. Hence, a thinner, more wear resistant, coating is a big advantage.
(Ti,Al,Cr,Si)N coatings on cutting tools are known in the art.
EP 1 219 723 A discloses a hard film for cutting tools composed of Ti1-a-b-c-dAlaCrbSicBd(C1-eNe) where 0.5<a<0.8, b>0.06, is 0≦c<0.1, 0≦d<0.1, 0<c+d<0.1 and 0.5<e<1. The hard film can be in the form of a multilayered structure.
US2006/0222893 discloses a multilayer coating comprising repeated layer stacks comprising at least one, 50-150 nm, (Al,Cr)N layer and/or at least one, 75-200 nm, (Ti,Si)N layer and at least one layer stack of (Al,Cr,Ti,Si)N+(Ti,Si)N+(Al,Cr,Ti,Si)N+(Al,Cr)N. The mixed (Al,Cr,Ti,Si)N layers have a multilayered structure and is achieved by running all targets simultaneously. This would lead to a coating with very fine layers, in the range of several nanometers. The (Al,Cr,Ti,Si)N layer have a thickness of 20±10 nm.